A desirable feature in certain graphics systems, such as a multi-media workstation, is the display of an interlaced image, such as a color television image, with a non-interlaced graphics display screen. However, several problems must be overcome in order to provide this capability in a satisfactory manner.
A first problem relates to enhancing the quality of the television image. One known method for providing image enhancement involves a digital filtering or decoding technique, as depicted in FIG. 1a. However, this technique requires real-time access, relative to the incoming video, to a neighborhood (A) of pixels (B) that are located on three consecutive television lines of a video field.
A second problem is related to accomplishing motion-adaptive deinterlacing. As illustrated in FIG. 1b this technique requires an access to pixels (B) located on three television lines. Two of the three line belong to a current video field while the third line, shown as a dashed line, is associated with a previous video field. Additionally, all three of the lines must be synchronized with the image lines of the graphics display.
A third problem is related to achieving a full synchronization of the deinterlaced television image relative to the graphics image. Such synchronization implies that fully stored frames, each frame comprising two fields, of television video must be used for the visualization of the television image on the graphics screen.
Aspects of the second and third problems are further illustrated in FIGS. 1c and 1d. FIG. 1c shows a position of an object, represented by a vertical line, in two consecutive television fields when the object is moving in a horizontal direction. The first field is depicted in FIGS. 1c(a) while the second, consecutive field is depicted in FIG. 1c(b). It can be seen that due to the horizontal offset between the object image in the two fields that if all lines of both television fields are displayed simultaneously on a graphics screen, as depicted in FIG. 1c(c), the image of the object is blurred.
FIG. 1d illustrates the case, for the same moving vertical object, when the graphics screen is not synchronized with incoming television video. As a result, if a new field is partially written into a frame buffer that contains a previous field the image of the moving object is split. FIG. 1d(c) illustrates a combined effect of deinterlacing and image splitting. As can be seen the net result is a blurring of the television image when displayed upon the non-interlaced graphics display screen.
In U.S. Pat. No. 4,694,325, issued Sep. 15, 1987, S. Mehrgardt discloses an interface circuit for interfacing a color television receiver to a home computer wherein the home computer has a graphics clock signal unsynchronized with that of the television receiver. The circuitry of Mehrgardt includes a digital delay line having cascaded delay stages. However Mehrgardt is concerned only with already decoded Red, Green and Blue signals and not with the reception and subsequent display of a composite signal.
In U.S. Pat. No. 4,344,075, issued Aug. 10, 1982, J. Rudy discloses a system for eliminating ragged vertical edges displayed by a NTSC color carrier on a non-interlaced display. Rudy discloses timing control circuitry that is operative only during a selected single scan line of a non-display portion of each successive field of a given non-interlaced television raster scan-line pattern (col. 2, lines 45-61).
In U.S. Pat. No. 4,698,674, issued Oct. 6, 1987, L. Bloom discloses a data converter for converting sequentially digitized interlaced data, from a television camera or other data source, to non-interlaced data for storage in a computer memory. The approach of Bloom is to store two fields of the image in a memory. Bloom apparently assumes that the fields of the television image are synchronized (gen-locked) with the frames of the graphics screen. However, in practically all applications this is not the case. Instead, the timing of a non-interlaced graphics controller is typically totally independent from a television video source.
Other references of general interest include the following. In U.S. Pat. No. 3,970,776, Jul. 20, 1976 K. Kinuhata et al. disclose a system for converting the number of lines of a television signal having interlaced frames, each frame formed by two adjacent interlaced fields. In U.S. Pat. No. 4,484,188, issued Nov. 20, 1984 G. Ott discloses video signal generation circuitry for improving the resolution of a video signal by forming additional video scan lines between successive scan lines. The system of Ott forms the additional video scan line by combining video attributes of adjacent scan lines In U.S. Pat. No. 4,480,267, issued Oct. 30, 1984, P. van den Avoort et al. disclose field interpolation for obtaining substantially equal amplitudes of information from each of two successive fields of a television signal. van den Avoort is concerned with a conversion from a 313-line television picture to a 625-line television picture. In U.S. Pat. No. 4,694,348, issued Sep. 15, 1987, K. Kamiya et al. disclose a scan interlace converter for a liquid crystal display panel of a television receiver. In U.S. Pat. No. 4,660,070, issued Apr. 21, 1987, Nishi et al. disclose a video display processor for writing video image data into a video memory. The video display processor generates memory address data in accordance with horizontal and vertical synchronization signals. In U.S. Pat. No. 4,518,984, issued May 21, 1985 H. Mitschke discloses circuitry including a video frame store 221 for obtaining a flicker-free image when producing a combined text and graphics (Videotext) display.
However, none of the forgoing U.S. Patents either singularly or in combination teach a method or apparatus that overcomes in a satisfactory manner all of the above described problems relating to image quality enhancement, motion-adaptive deinterlacing and synchronization when displaying an interlaced image, such as a television signal, with a non-interlaced graphics display system.
It is thus an object of the invention to provide method and apparatus for displaying an interlaced image signal on a non-interlaced graphics display screen.
It is a further object of the invention to provide method and apparatus for displaying on a non-interlaced graphics display screen a television color image frame comprised of two interlaced fields; the invention providing three field buffers, two for storing the two fields of a completed frame while the third field buffer stores a current television field.